JP2913898B2 - Engine exhaust purification system - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust gas purification system for removing a large amount of unburned hydrocarbons immediately after starting an engine.

[0002]

2. Description of the Related Art In order to purify engine exhaust gas,
Usually, a catalyst is provided in the exhaust passage. But,
The catalyst does not operate effectively until it reaches a temperature of about 300 ° C. or higher. Therefore, immediately after the start of the engine, that is, when the exhaust gas temperature is low, the exhaust gas cannot be sufficiently purified. On the other hand, immediately after the start of the engine, the engine temperature is low, and a large amount of unburned hydrocarbons is discharged. Therefore, a pre-catalyst is provided in the exhaust gas upstream of the main catalyst, that is, in the exhaust passage close to the engine so that the catalyst reaches the operating temperature as soon as possible, and the unburned hydrocarbon discharged immediately after the engine is started is purified by the pre-catalyst. A method has been proposed (Japanese Unexamined Patent Publication No.
134711 publication).

[0003]

By providing a pre-catalyst in the exhaust passage close to the engine, the efficiency of purifying a large amount of unburned hydrocarbons discharged immediately after the start of the engine is improved. To comply with regulations, it is necessary to further improve purification efficiency. Therefore, as a result of a detailed study of the temperature rise of the pre-catalyst, it was found that the temperature rise speed of the pre-catalyst was much slower than that of the exhaust gas. After a thorough study of the cause, we concluded that liquid water contained in the exhaust gas was captured by the pre-catalyst, and the heat for vaporizing this water (latent heat of evaporation) was taken from the catalyst. .

[0004] It is an object of the present invention to improve the efficiency of purifying a large amount of unburned hydrocarbons discharged immediately after the engine is started.

[0005]

According to the present invention, in order to achieve the above-mentioned object, a water retention material is provided on the upstream side of a pre-catalyst to temporarily hold liquid water, and water adheres to the pre-catalyst. Was suppressed.

[0006]

In order to assist the function of the main catalyst provided in the exhaust passage of the engine, a pre-catalyst is disposed near the engine on the exhaust side of the main catalyst, and the distribution of the exhaust gas temperature from the start of the engine, The catalyst temperature and the exhaust gas purification rate (particularly unburned hydrocarbons) were examined. As a result, it became clear that the exhaust gas temperature at the inlet of the pre-catalyst immediately reached nearly 300 ° C., while it took time for the pre-catalyst temperature and the exhaust temperature at the pre-catalyst outlet to rise to 300 ° C. Examination of the cause revealed that liquid water contained in the exhaust gas adhered to the precatalyst, and this water took away the heat of the exhaust gas and evaporated. When the engine was started, the temperature of the exhaust passage was low, and it was considered that the exhaust gas was cooled by the inner wall of the exhaust passage, water vapor was condensed, and water droplets adhered to the precatalyst. Therefore,
Temporarily catching water droplets generated near the inner wall of the exhaust passage,
An attempt was made to avoid sticking to the precatalyst.

[0007] As a method, a water retention material is provided on the upstream side of the pre-catalyst. Water droplets are considered to be generated particularly near the inner wall of the exhaust passage where the exhaust is cooled, and are arranged along the inner wall of the exhaust passage to prevent a large pressure loss. As the moisture retaining material, a stainless wire or a stainless steel plate was installed along the inner wall so that the inner wall had irregularities. Water droplets are held in the concave portions. Also, by placing a porous metal or porous ceramic on the inner wall,
Water droplets could be retained in the holes. That is, when the exhaust passage is not sufficiently warmed, the generated water droplets are captured by the moisture retaining material, and the captured water droplets evaporate as the exhaust passage temperature increases. At this time, since the exhaust gas temperature is already sufficiently high, the catalyst temperature does not become lower than the operating temperature even when the exhaust gas is cooled. In addition, a part of the moisture captured by the moisture retaining material enters the pre-catalyst as water droplets. However, since the amount of heat required to evaporate the water is small, the temperatures of the pre-catalyst and the exhaust temperature at the outlet of the pre-catalyst slightly decrease. Only. From these facts, by providing the water retention material on the upstream side of the pre-catalyst, the purification rate of unburned hydrocarbons discharged immediately after the start of the engine is greatly improved.

[0008]

<Embodiment 1> A three-way catalyst 3 (capacity: 1.0) usually used in an automobile is provided in an exhaust passage 2 of a gasoline engine 1.
l), and a pre-catalyst 4 (volume 0.3)
l) was provided. An oxidation catalyst was used as a precatalyst. The moisture retaining material 5 was arranged on the upstream side of the pre-catalyst. A side view is shown in FIG. A stainless wire (3 mm outside diameter) was wound around the moisture retaining material along the inner wall of the exhaust passage. FIG. 2 shows a cross-sectional view of the moisture retaining material. Wound so that the distance between the centers of adjacent stainless steel wires is 7 mm. The length of the part around which the stainless wire was wound was about 100 mm.

Example 2 Porous ceramic was used as a moisture retaining material. The arrangement of the device is the same as in the first embodiment. The porous ceramic was used by laminating alumina cloth. FIG. 3 shows the structure of the moisture retaining material. The alumina cloth used was a plain woven product having a warp and a weft of 11 yarns / 25 mm and a thickness of 0.7 mm. Four of these were stacked and attached to the inner wall of the exhaust passage 100 mm.

Example 3 A porous metal was used as a moisture retaining material. The arrangement of the device is the same as in the first embodiment. Stainless steel mesh was used as the porous metal. The structure of the moisture retaining material is the same as in FIG. Stainless steel mesh
Five sheets each having a wire diameter of 0.2 mm and an opening of about 0.25 mm were used. This was mounted on the inner wall of the exhaust passage 100 mm.

<Comparative Example 1> The moisture retaining material 5 was removed from the apparatus shown in Example 1.

Test Example 1 In order to examine the exhaust gas purification performance of the engine exhaust gas purification system shown in Examples 1, 2, 3 and Comparative Example 1, a cold start test was carried out using an engine bench. That is, starting, idling, acceleration, 60 km
/ H running, and the unburned hydrocarbons discharged were analyzed.
The purification performance of the engine exhaust purification system was evaluated by comparing the total amount of unburned hydrocarbons released during the test. FIG. 4 shows the results. The vertical axis in the figure is a value when the total amount of unburned hydrocarbons released in the case of Comparative Example 1 is 1.
As is apparent from this figure, the provision of the water retention material on the upstream side of the pre-catalyst improves the purification efficiency of the catalyst,
The amount of unburned hydrocarbons released is reduced to about 2/3.

[0013]

According to the present invention, the temperature of the pre-catalyst reaches the operating temperature quickly by temporarily holding the liquid water condensed in the exhaust gas at the time of starting the engine before the pre-catalyst. The purification efficiency of the discharged hydrocarbon can be improved.

[Brief description of the drawings]

FIG. 1 is a side view of an engine exhaust gas purification system according to an embodiment of the present invention.

FIG. 2 is a cross-sectional view illustrating a structure of a moisture retaining material.

FIG. 3 is an explanatory diagram of a moisture retaining material according to a second embodiment of the present invention.

FIG. 4 is an explanatory view showing the unburned hydrocarbon purification performance of FIG. 1;

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Engine, 2 ... Exhaust passage, 3 ... Three-way catalyst, 4 ... Pre-catalyst, 5 ... Water retention material.

──────────────────────────────────────────────────続 き Continuing on the front page (72) Osamu Kuroda 4026 Kuji-cho, Hitachi, Hitachi, Ibaraki Hitachi, Ltd.Hitachi Research Laboratory (72) Inventor Toshio Yamashita 4026 Kuji-cho, Hitachi, Ibaraki Hitachi, Ltd.Hitachi Research In-house (72) Inventor Hiroshi Miyadera 4026 Kuji-cho, Hitachi City, Ibaraki Prefecture Inside Hitachi Research Laboratory, Hitachi, Ltd. (56) References JP-A-3-179122 (JP, A) JP-A-2-207120 (JP, A) Japanese Utility Model Application Showa 60-190923 (JP, U) Japanese Utility Model Application Hei 2-91629 (JP, U) (58) Fields investigated (Int. Cl. ⁶ , DB name) F01N 3/20 F01N 3/24

Claims (5)

(57) [Claims] A main catalyst for purifying exhaust gas is disposed in an exhaust passage of an engine.
In an engine exhaust gas purification system in which a pre-catalyst for purifying discharged unburned hydrocarbons is provided, a water retention unit is provided on the upstream side of the pre-catalyst, and liquid water contained in exhaust immediately after the start of the engine is temporarily removed. An engine exhaust purification system characterized in that: 2. The method according to claim 1, wherein the water holding portion is porous.
Engine exhaust purification system that is composed by the quality inorganic material. 3. The engine exhaust gas purification system according to claim 1 , wherein said water holding portion is made of a porous metal or a porous ceramic . 4. The method according to claim 1 , wherein the water holding portion is made of a metal.
An engine exhaust purification system formed of a plate or metal wire . 5. The engine exhaust gas purification system according to claim 1, wherein said moisture retaining portion is provided on an inner wall of an exhaust passage.